Optical fiber filter of wideband deleterious light and uses thereof

ABSTRACT

Optical fiber filters and uses thereof are presented. In typical implementations, there is provided a FBG taking deleterious light out of a fiber core without reflecting it into the fiber core. It also allows the unhindered transmission of useful light at a wavelength outside of the spectral band covered by the deleterious light. The filter couples the incoming deleterious light to cladding modes propagating in the opposite direction without coupling the incoming useful light to core or cladding modes propagating in the opposite direction. The filter may for example be useful as a Raman or ASE filter in a laser cavity of other optical devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. application Ser. No.16/515,132, filed on Jul. 18, 2019, now U.S. Pat. No. 10,663,654, whichis a continuation of U.S. application Ser. No. 15/880,834, filed on Jan.26, 2018, now U.S. Pat. No. 10,393,955, which claims the benefit under35 U.S.C. § 119 to U.S. Provisional Patent Application No. 62/451,095,filed on Jan. 27, 2017, and to Canadian Patent Application No.2,971,601, filed on Jun. 23, 2017, the entireties of which areincorporated herein by reference.

TECHNICAL FIELD

The technical field generally relates to optical fiber devices.

BACKGROUND

The optical fiber has found widespread use because of its extraordinaryability to guide light over considerable distances with littleattenuation. This ability is typically maintained over a sizablespectral range, a feature which in some applications becomes troublesomeby allowing guidance of deleterious light.

A first example of such an application is the generation of short pulsesof light with Q-switched fiber lasers. As known in the art, amplifiedspontaneous emission (ASE) constitutes an important limiting factor insuch lasers [M. Morin et al., Q-switched Fiber Lasers, Chapter 7 inRare-Earth-Doped Fiber Lasers and Amplifiers, Second Edition, Revisedand Expanded, M. J. F. Digonnet ed., Marcel Dekker, 341-394 (2001)]. Theoperation of a Q-switched laser involves a pumping stage during whichlaser oscillation is impeded by strong intra-cavity optical losses. Theobjective of the pumping stage is to store as much energy as possiblewithin the gain medium, thus realizing as well the highest possiblegain. The intra-cavity losses are then turned off suddenly, either byexternal means (active Q-switching) or by the optical power itself(passive Q-switching). The high gain coupled to the low cavity lossesensures a very rapid build-up of the laser oscillation leading to theformation of a short and energetic optical pulse. The ASE taking placeduring the pumping stage can lead to a sizable depletion of the energystored in the gain medium, thus lowering the available energy and thegain at the time the intra-cavity losses are turned off. This leads toweaker and longer pulses. Wideband intra-cavity attenuation of the ASEis therefore desirable to keep it from reaching appreciable levels.

Amplified spontaneous emission can be troublesome in cw (continuouswave) fiber lasers as well [Y. Glick et al., Single Mode 1018 nm fiberlaser with power of 230W, Proc. SPIE 9728, 97282T (2015)]. From athermal management point of view, it is preferable to run a high-powerfiber laser at the shortest wavelength afforded by the gain medium. Thisreduces the Stokes defect between pump and laser photons, i.e. thedifference between the energy of each pump photon absorbed to excite thegain medium and that of each photon emitted by the laser. Globally,running the laser at a shorter wavelength reduces the difference betweenthe pump power absorbed by the gain medium and the optical power emittedby the laser. This difference comes out as heat that must be dissipatedsomehow. Operation at a short wavelength is difficult when the availablegain at longer wavelengths is stronger. Narrow band reflectors, thatprovide feedback at the shorter wavelengths but not at longerwavelengths, can be used to avoid lasing at the longer wavelengths. Evenin this case, the stronger gain at longer wavelengths can lead to apowerful emission of ASE and a sizable reduction of the laser emissionat the shorter wavelength. Such lasers can thus benefit from theintra-cavity wideband filtering of ASE as well.

The optical fiber, by allowing the propagation of intense light overlong distances, is an ideal medium for observing nonlinear effects. Oneof these effects is Raman scattering, resulting from the interactionbetween an intense optical field and the glass molecules constitutingthe fiber [G. P. Agrawal, Nonlinear Fiber Optics 2^(nd) ed., AcademicPress, Chapter 8, 316-369 (1995)]. Raman scattering manifests itself asa transfer of power from an incoming optical wavelength to a longerwavelength, the spectral shift being characteristic of the materialwhere it occurs. In fused silica, the Raman gain extends over tens ofnanometers and is maximum at a wavelength shift of 46 nm when theincident light has a wavelength of 1000 nm. Raman scattering can be aserious impediment in various applications. It limits the reach ofoptical fiber communication links that can be achieved by increasing theoptical power of the signal launched in the fiber. When the opticalpower reaches a threshold value (see e.g. G. P. Agrawal, Nonlinear FiberOptics 2^(nd) ed. supra, section 8.1.2), Raman scattering sets in andleads to a sizable transfer of power to longer wavelengths [J.-P.Blondel et al., Elimination of optical power limitation due tostimulated Raman scattering in fiber optic links, U.S. Pat. No.6,529,672]. This is clearly problematic in optical communications linkswhere each channel is carried by a given wavelength. Raman scattering issignificant when watt-level optical powers propagate over kilometers ofsingle mode fiber. In fiber laser systems operating at kilowatt-leveloptical powers, Raman scattering sets in over commensurably shorterfibers. It contributes to a detrimental spectral widening of the laseroutput beam [T. Schreiber et al., Analysis of stimulated Ramanscattering in cw kW fiber oscillators, Proc. SPIE 8961, 89611T (2014)].The main application of high power fiber lasers is material processing.A high-power fiber laser and the optical link carrying the laser outputare designed to get the laser output to a target with minimum losses.One motivation is to maximize the efficiency of the material processing.Another is reliability and security, ensuring that the high opticalpower does not go where it should not. Light generated by Ramanscattering, being at a sizably different wavelength than that generatedby the laser gain medium, can interact differently than designed forwith mirrors, filters, optical coatings and optics, reducing efficiencyand raising reliability and security concerns. Raman scattering can takeplace in the fiber laser itself but also in optical fiber links coupledto the laser. Given the high optical powers involved, the lightgenerated by Raman scattering can become quite powerful. Back reflectionof this powerful Raman light in the fiber laser can destabilize itsoperation and even lead to optical damage [V. P. Gapontsev et al.,Method and apparatus for preventing distortion of powerful fiber-lasersystems by backreflected signals, U.S. Pat. No. 7,912,099]. Ramanscattering is a major impediment limiting the achievable power in fiberlaser systems.

Diverse types of fibers have been proposed to either thwart thegeneration of wideband deleterious light or attenuate preferentiallywideband deleterious light. By way of example, in the case of Ramanscattering and other nonlinear effects, one approach is to increase thetransversal extent over which light is carried by an optical fiber, thusreducing the optical intensity (W/cm²) for a given optical power (W). Avast body of technical literature is devoted to large mode area (LMA)fibers, i.e. fibers with a transversal structure that supports a largerfundamental core mode (see e.g. [J. M. Fini, Large-mode-area opticalfibers with reduced bend distortion, U.S. Pat. No. 7,783,149] andreferences found therein). However, there is a practical limit to thisapproach as the sensitivity of a fiber to bending typically increaseswith the fundamental mode effective area. Still larger core fibers canbe used that support multiple core modes, but at the expense of areduction in the optical quality of light carried by the fiber, which isthen more difficult to focus to a tight spot. Coiling slightly multimodefibers can be used to attenuate preferentially higher order modes[Selecting the Optimal LMA Fiber, Application Note NuAPP-2, Nufern].Various optical fibers have also been proposed that provide preferentialattenuation over specific wavelength bands (see e.g. S. G. Grubb et al.,Optical fiber gain medium with evanescent filtering, U.S. Pat. No.6,118,575; R. T. Bise et al., Optical fiber for suppression of amplifiedspontaneous emission, U.S. Pat. No. 7,272,287; T. Tam and J. C. Knight,Optical power delivery system, U.S. Pat. No. 7,643,715; R. Goto,Photonic bandgap fiber, U.S. Pat. No. 8,035,891; A. Petersson et al.,Active optical fibers with wavelength-selective filtering mechanism,method of production and their use, U.S. Pat. No. 8,045,259; T. Tam etal., All solid photonic bandgap fiber, U.S. Pat. No. 8,503,846; J. M.Fini et al., Distributed suppression of stimulated Raman scattering inan Yb-doped filter-fiber amplifier, Optics Letters 31, 2550-2552 (2006);and J. Kim et al., Suppression of stimulated Raman scattering in a highpower Yb-doped fiber amplifier using a W-type core with fundamental modecut-off, Optics Express 14, 5103-5113 (2006), and references therein).Some of these fibers must be bent or coiled to perform as desired (seee.g. J. W. Nicholson et al., Filter fiber for use in Raman lasingapplications and techniques for manufacturing same, U.S. Pat. No.8,428,409; and J. M. Fini and J. W. Nicholson, Optical fiber withdistributed bend compensated filtering, U.S. Pat. No. 9,322,989). Theconcatenation of fibers with different core sizes, including tapered andbent fiber segments, has also been proposed to provide filtering ofdeleterious light (M. P. Savage-Leuchs, Method and apparatus for opticalgain fiber having segments of differing core sizes, U.S. Pat. No.7,768,700; M. P. Savage-Leuchs, Apparatus and method for optical gainfiber having segments of differing core sizes, U.S. Pat. No. 8,089,689;M. P. Savage-Leuchs, Optical gain fiber having segments of differingcore sizes and associated method, U.S. Pat. No. 8,199,399; M. P.Savage-Leuchs, Method and optical gain fiber having segments ofdiffering core size, U.S. Pat. No. 8,345,348; and M. P. Savage-Leuchs,Optical gain fiber having tapered segments of differing core sizes andassociated method, U.S. Pat. No. 8,705,166). Self-imaging in a multimodeinterference filter (MMI) can be used to filter out an undesirablewavelength [V. P. Gapontsev et al., U.S. Pat. No. 7,912,099, supra]. Tothis end, a segment of multimode fiber is inserted between two singlemode fibers. Properly adjusting the length of the multimode fibersegment ensures that a useful wavelength is transmitted with little losswhile other wavelengths get attenuated.

Other than performance limitations (see e.g. J. Kim et al., OpticsExpress 14, (2006), supra; F. Jansen et al., Modeling the inhibition ofstimulated Raman scattering in passive and active fibers by lumpedspectral filters in high power fiber laser systems, Optics Express 17,16255-16265 (2009); and D. Nodop et al., Suppression of stimulated Ramanscattering employing long period gratings in double-clad fiberamplifiers, Optics Letters 35, 2982-2984 (2010) for a discussion), adisadvantage of these approaches is their reliance on specific fiberdesigns. Their implementation requires the insertion of one or multiplesegments of fiber within a system. A fiber that is optimal for filteringmay not be optimal for other aspects of a system operation. Moreover,these approaches afford little flexibility as the potential performanceis predetermined by the fiber design.

Referring to FIG. 3 (PRIOR ART) it is also known to use a uniform periodFBG coupled to a circulator to separate useful light from widebanddeleterious light. Light enters the circulator through a first port. Itthen reaches the second port of the circulator where the FBG isconnected. Deleterious light at wavelengths outside of the reflectivityspectrum of the FBG is transmitted and leaves the circulator through thesecond port. Deleterious light at shorter wavelengths than the usefullight can also be reflected into cladding modes (not indicated in thefigure). Useful light is reflected by the FBG into the fiber core andtowards the third port of the circulator. This approach requires asupplementary optical component (the circulator). Transmission throughthe circulator and a less than 100% reflectivity of the FBG can bothinduce losses to the useful light. Furthermore, this approach is notwell adapted to situations involving high peak powers or high averagepowers because of the risk of damage to the circulator, either opticalor thermal.

FBGs having a chirped period (CFBGs), slanted fringes (SFBGs) or both(CSFBGs) are known in the art of light filtering. Gain flattening inoptical fiber communications link has been a major application ofCSFBGs, the optical loss of a CSFBG combining with the gain of anamplifier to provide an effective amplification that is uniform over aspectral band of interest [I. Riant and P. Sansonetti, Filter opticalwaveguide with inclination and linear chirp, U.S. Pat. No. 6,321,008].CSFBGs have also been used to attenuate light over the spectral band1520-1565 nm in order to favor amplification over the spectral band1565-1625 nm in L-band Er-doped fiber amplifiers [R. P. Espindola etal., Article comprising an L-band optical fiber amplifier, U.S. Pat. No.6,141,142]. The suppression of Raman scattering in optical fibers withlumped filters is discussed in J.-P. Blondel et al., U.S. Pat. No.6,529,672 (supra) and F. Jansen et al., Optics Express 17, (2009)(supra), both references addressing the optimal positioning of multiplefilters along an optical fiber to impede the growth of Raman scattering.Blondel et al stresses the importance of filtering both forward andbackward propagating Raman light and the importance of avoidingreflection of light in the fiber core by the lumped filters. Jansen etal proposed using long period gratings (LPG) for filtering. This wasfollowed by an experimental demonstration of the suppression of Ramanscattering in a fiber amplifier using LPGs as filters [D. Nodop et al.,Optics Letters 35, (2010), (supra)]. Filtering in a LPG and in a SFBGresults from coupling light from the core and into the cladding, thedifference being that a LPG transmits light into the cladding whereas aSFBG reflects light into it. In both cases, light coupled into thecladding is eventually lost. Gapontsev et al. (supra) discloses the useof SFBGs in a high-power MOPA system to avoid a powerful and potentiallydestructive reflection of Raman light into a fiber laser oscillator. D.A. V. Kliner and T. S. McComb, Slanted FBG for SRS suppression, USpatent application 20160111851 discloses the suppression of Ramanscattering with a SFBG that is explicitly chirped.

The suppression of deleterious light inside a laser cavity has beenconsidered as well. J. Liu (Hybrid high power laser to achieve highrepetition rate and high pulse energy, US patent application20060029111) discloses the insertion of FBGs inside a laser cavity,without specifying further the nature of the gratings, to reduce ASE andRaman scattering. H. Po and A. A. Demidov, Multi-wavelength opticalfiber, U.S. Pat. No. 7,340,136 discloses the use of LPGs and SFBGs in aRaman laser to suppress the generation of a given Stokes order. In aRaman laser, a cascade of cavities is used to generate light of evergreater wavelength. A first cavity is built to resonate at thewavelength of a pump light. The ensuing high intensity of the pump lightleads to the generation of light at a longer wavelength through Ramanscattering. This light at a longer wavelength, called the first Stokesorder, is used to pump a second cavity designed to resonate at thelonger wavelength. The ensuing high intensity at the longer wavelengthfavors the generation of light at a still longer wavelength throughRaman scattering, called the second Stokes order, and so on. H. Po etal. (supra) discloses the insertion of a LPG or SFBG in a cavity tosuppress Raman scattering past a desired maximum Stokes order. Eventhough the origin of the gain sustaining oscillation is different thanin a standard laser, the general idea is the same, i.e. the introductionof a filter inside a cavity to impede the generation of undesirablelight. Kliner et al. (supra) discloses the insertion of a CSFBG inside alaser cavity to suppress Raman scattering.

To prevent the reflection of light into counter-propagating core modes,SFBG with a pronounced tilt angle of the grating fringes are preferablyused [R. Kashyap et al., Wideband gain flattened erbium fibre amplifierusing a photosensitive fibre blazed grating, Electronics Letters 29,154-156 (1993)]. However, as discussed in Riant et al. (supra), a largertilt angle makes it more difficult to precisely define the spectralresponse of a CSFBG. The realization of SFBGs producing littlereflection in the fiber core has received quite a bit of attention (seee.g. T. A. Strasser and P. S. Westbrook, Article comprising a tiltedgrating in a single mode waveguide, U.S. Pat. No. 6,427,041; andreferences found therein). The reduction in reflectivity is achieved byusing optical fibers with specially tailored refractive index andphotosensitivity profiles (I. Riant et al. (supra); A. Strasser et al.,(supra); I. Riant and C. De Barros, Optical waveguide and method forcreating an asymmetrical optical filter device, U.S. Pat. No. 7,035,515;S. Ishikawa et al., Optical fiber and fiber grating type filterincluding the same, U.S. Pat. No. 7,203,399; C. De Barros et al.,Photosensitive optical waveguide, U.S. Pat. No. 7,389,022). Theseprofiles are designed to minimize the scattering efficiency between thecounter-propagating fundamental core modes. Another approach is toperform the mode coupling in a slightly multimode fiber with a SFBGdesigned to couple the fundamental core mode with a higher order coremode [C. De Barros et al., Optical filter, U.S. Pat. No. 7,095,924]. Theslightly multimode fiber is inserted between two single mode opticalfibers that do not support the higher order core mode. A disadvantage ofthese approaches is again their reliance on specific optical fibers.

There remains a need for efficient filtering of deleterious light inoptical fiber devices while alleviating at least some of the drawbacksof the prior art.

SUMMARY

The present description is concerned with the wideband filtering ofdeleterious light propagating in an optical fiber, this filtering beingrealized without producing any significant reflection of light withinthe fiber core.

The present description generally concerns a filter inscribed within anoptical fiber. In typical implementations, this filter takes deleteriouslight covering a wide spectral band out of the fiber core withoutreflecting it into the fiber core. It also allows the unhinderedtransmission of useful light at a wavelength outside of the spectralband covered by the deleterious light. More specifically, incomingdeleterious light is carried by core modes of the fiber. The filtercouples the incoming deleterious light to cladding modes propagating inthe opposite direction without coupling the incoming deleterious lightto core modes propagating in the opposite direction. Likewise, incominguseful light is carried by core modes. The filter transmits the usefullight without coupling it to either core modes or cladding modespropagating in the opposite direction. These are ideal characteristicsthat the present filter, when properly designed, can fulfill closely.Such a filter is suitable for use inside a laser cavity or in situationswhere minute reflections are problematic, as in the presence of highgain Raman scattering or the like.

In accordance with one aspect, there is provided a Raman filter forfiltering a light beam having a useful component and a Raman component.The Raman filter includes an optical fiber path having a core and atleast one cladding surrounding the core. At least one Fiber BraggGrating (FBG) having an input end and an output end is disposed alongthe optical fiber path to receive the light beam along a core mode atthe input end. The FBG includes a refractive index modulation in thecore of the optical fiber path. The refractive index modulation definesslanted grating fringes having a tilt angle and a longitudinal variationdefining a chirped grating period which is maximum at the input end anddecreases progressively from the input end to the output end. Thegrating period further has a variation defining a Bragg wavelengthlonger than a wavelength of the useful component at all points along theFBG.

In some implementations, the tilt angle of the grating fringes isbetween about 1.5 and 15 degrees, or between 2.6 and 5.2 degrees. Thetilt angle of the grating fringes may vary along the FBG.

In some implementations, the refractive index modulation defines gratingfringes covering a portion only of the core. These grating fringes maybe normal to a longitudinal axis of said core.

In some implementations, the variation of the grating period and thetilt angle of the grating fringes are designed such that a shorterwavelength providing coupling to cladding modes is longer than thewavelength of the useful component at all points along the FBG.

The core of the optical fiber path may be multimode.

The grating period may be linearly chirped or nonlinearly chirped.

In some implementations, the refractive index modulation has an apodizedamplitude.

In some implementations, the Raman filter includes two FBGs disposedoutput end-to-output end along the optical fiber path. The optical fiberpath hosting the two FBGs may be a single segment of optical fiber, ortwo segments of optical fiber optically coupled together, each of thetwo segments of optical fiber hosting one of said two FBGs. In somevariants, the chirped grating period of the respective grating fringesof the two FBGs have a same longitudinal variation.

In some implementations, the Raman filter may further include an activetuning mechanism coupled to the FBG.

In accordance with another aspect, there is provided an optical fiberpath for transporting a light beam generating Raman scattered light.

The optical fiber path includes a core carrying the light beam, at leastone cladding surrounding the core, and a Fiber Bragg Grating (FBG)having an input end and an output end and disposed along the opticalfiber path to receive the light beam along a core mode at the input end.

The FBG includes a refractive index modulation in the core of theoptical fiber path having a profile designed to allow the light beam topropagate to the output end while coupling the Raman scattered lightinto one or more counter-propagating cladding mode. The refractive indexmodulation has a period maximum at the input end and decreasingprogressively from the input end to the output end.

The period of the reflective index modulation may be linearly chirped ornonlinearly chirped.

In some implementations, the refractive index modulation defines slantedgrating fringes having a tilt angle. The tilt angle of the gratingfringes may be between about 1.5 and 15 degrees, or between 2.6 and 5.2degrees. The tilt angle of the grating fringes may vary along the FBG.

In some implementations, the refractive index modulation defines gratingfringes covering a portion only of the core. These grating fringes maybe normal to a longitudinal axis of said core.

In some implementations, the refractive index modulation may have anapodized amplitude.

In accordance with another aspect, there is provided an optical devicesupporting a light beam having a useful component and a deleteriouscomponent. The optical device includes an optical fiber path having acore and at least one cladding, and configured to guide the light beamin a core mode along a propagation direction. The optical device furtherincludes a Fiber Bragg Grating (FBG) disposed along the optical fiberpath and having an input end and an output end with respect to thepropagation direction. The FBG includes a refractive index modulation inthe core of the optical fiber path having a chirped period larger at theinput end than at the output end. The refractive index modulation isconfigured to allow the useful component of the light beam through theFBG and to couple the deleterious component of the light beam into oneor more counter propagating cladding mode of the optical fiber path.

In some implementations, the refractive index modulation defines slantedgrating fringes having a tilt angle. The tilt angle of the gratingfringes may be between about 1.5 and 15 degrees, or between 2.6 and 5.2degrees. The tilt angle of the grating fringes may vary along the FBG.

In some implementations, the refractive index modulation defines gratingfringes covering a portion only of the core. These grating fringes maybe normal to a longitudinal axis of said core.

In some implementations, the core of the optical fiber path ismultimode.

The period of the refractive index modulation may be linearly ornonlinearly chirped.

In some implementations, the refractive index modulation has an apodizedamplitude.

In some implementations, an active tuning mechanism may be coupled tothe FBG.

In some implementations, the optical device is a laser including a lasercavity. The FBG may be positioned inside or outside of the laser cavity.

In some implementations, the deleterious component may be one ofAmplified Spontaneous Emission and Raman scattering.

In accordance with another aspect, there is provided a method offiltering a Raman component out of a light beam having a usefulcomponent and said Raman component. The method includes the steps of:

-   -   designing a modulation index profile defining a Bragg Grating        having an input end and an output end, the refractive index        modulation including slanted grating fringes having a tilt angle        and a longitudinal variation defining a chirped grating period        which is maximum at the input end and decreases progressively        from the input end to the output end. The grating period has a        variation defining a Bragg wavelength longer than a wavelength        of the useful component at all points along the FBG;    -   photoinducing the modulation index profile in a core of an        optical fiber path, the modulation index profile being        positioned along the optical fiber path so as to receive the        light beam along a core mode at the input end; and    -   propagating the light beam along the optical fiber path.

In some implementations, the tilt angle of the grating fringes isbetween about 1.5 and 15 degrees. In some implementations, the tiltangle of the grating fringes is between 2.6 and 5.2 degrees.

In some implementations, the designing of the refractive indexmodulation profile includes varying the grating period and the tiltangle of the grating fringes such that a shorter wavelength that getscoupled to cladding modes in the optical fiber path is longer than thewavelength of the useful component at all points along the FBG.

The grating period may be linearly or nonlinearly chirped.

In some implementations, the refractive index modulation profile has anapodized amplitude.

In accordance with another aspect, there is provided a bidirectionalfilter for filtering a light beam having a useful component and adeleterious component.

The bidirectional filter includes a pair of Fiber Bragg gratings (FBG).Each FBG includes an optical fiber path having a core and at least onecladding surrounding this core, and a refractive index modulation in thecore of the optical fiber path and having a chirped period.

The FBGs of the pair are disposed end-to-end with the period of thecorresponding refractive index modulation decreasing progressivelytowards the other one of the FBGs. The refractive index modulation ofeach FBG is configured to allow propagation of the useful component ofthe light beam through the FBG in a core mode and to couple thedeleterious component of the light beam into one or more counterpropagating cladding mode of the optical fiber path.

In some implementations, the refractive index modulation of each FBGdefines slanted grating fringes having a tilt angle. The tilt angle ofthe grating fringes may be between about 1.5 and 15 degrees, or between2.6 and 5.2 degrees. The refractive index modulation may also definegrating fringes covering a portion only of the core.

The core of the optical fiber path of each FBG may be multimode.

The period of the refractive index modulation of each FBG may belinearly or nonlinearly chirped.

In some implementations, the refractive index modulation of each FBG hasan apodized amplitude.

The deleterious component may for example be one of AmplifiedSpontaneous Emission and Raman scattering.

In some implementations, the bidirectional filter further includes anactive tuning mechanism coupled to at least one of the FBGs.

The refractive index modulations in the FBGs of the pair may have a sameprofile or different profiles.

In accordance with another implementation, there is also provided abidirectional filter for filtering a light beam having a usefulcomponent and a deleterious component. The bidirectional filter includesan optical fiber path having a core and at least one claddingsurrounding the core. A Fiber Bragg grating (FBG) having a refractiveindex modulation is provided along the core of the optical fiber pathand has a period varying along the FBG. This period is maximum atopposite ends of the FBG and decreases progressively towards a midpointthereof. The refractive index modulation of the FBG is configured toallow propagation of the useful component of the light beam through theFBG in a core mode and to couple the deleterious component of the lightbeam into one or more counter propagating cladding mode of the opticalfiber path.

In some implementations, the refractive index modulation defines slantedgrating fringes having a tilt angle. The tilt angle of the gratingfringes may be between about 1.5 and 15 degrees, or between 2.6 and 5.2degrees. In some other implementations the refractive index modulationdefines grating fringes covering a portion only of the core.

The core of the optical fiber path may be multimode.

The period of the refractive index modulation may be nonlinearlychirped, or may be linearly chirped between the midpoint of the FBG andthe respective output ends. In some variants, the refractive indexmodulation may have an apodized amplitude.

In some implementations, the deleterious component is one of AmplifiedSpontaneous Emission and Raman scattering.

In some implementations, the bidirectional filter may further include anactive tuning mechanism coupled to the FBG.

In accordance with another aspect, there is provided a fiber lasercavity.

The fiber laser cavity includes an optical fiber path having a core andat least one cladding surrounding the core, the optical fiber pathhaving opposite extremities A pair of cavity mirrors are disposed oneither extremity of the optical fiber path.

The fiber laser cavity further includes a Raman filter or an ASE filterdisposed in the optical fiber path in close proximity to one of thecavity mirrors. The Raman filter or the ASE filter includes a FiberBragg Grating (FBG) having an input end and an output end and disposedalong the optical fiber path with the output end on a side of theproximate one of the cavity mirrors. The FBG has a refractive indexmodulation in the core of the optical fiber path having a profiledesigned to allow a useful laser beam to propagate to the output endwhile coupling Raman scattered light or ASE into one or morecounter-propagating cladding mode. The refractive index modulation has aperiod maximum at the input end and decreasing progressively from theinput end to the output end.

In some implementations, the refractive index modulation defines slantedgrating fringes having a tilt angle, which may be between about 1.5 and15 degrees, or between 2.6 and 5.2 degrees. The tilt angle of thegrating fringes may vary along the FBG.

In some implementations, the variation of the period of the refractiveindex modulation and the tilt angle of the grating fringes are designedsuch that a shorter wavelength that gets coupled to cladding modes islonger than the wavelength of the useful component at all points alongthe FBG.

The period of the refractive index modulation may be linearly ornonlinearly chirped.

In some implementations, the refractive index modulation has an apodizedamplitude.

In accordance with another aspect, there is provided a fiber lasercavity including an optical fiber path having a core and at least onecladding surrounding the core, the optical fiber path having oppositeextremities.

The fiber laser cavity also includes a pair of cavity mirrors disposedon either extremity of the optical fiber path.

The fiber laser cavity further includes a Raman filter or an ASE filterdisposed in the optical fiber path between the cavity mirrors. The Ramanfilter or ASE filter includes a Fiber Bragg grating (FBG) having arefractive index modulation along the core of the optical fiber path andhaving a period varying along this FBG. The period is maximum atopposite ends of the FBG and decreasing progressively towards a midpointthereof. The refractive index modulation is configured to allowpropagation of a useful laser beam through the FBG in a core mode and tocouple Raman scattered light or ASE into one or more counter propagatingcladding mode of the optical fiber path.

In some implementations, the refractive index modulation defines slantedgrating fringes having a tilt angle. The tilt angle of the gratingfringes may be between about 1.5 and 15 degrees, or between 2.6 and 5.2degrees. The tilt angle of the grating fringes may vary along the FBG.

In some implementations, the variation of the period of the refractiveindex modulation and the tilt angle of the grating fringes are designedsuch that a shorter wavelength providing coupling to cladding modes islonger than the wavelength of the useful component at all points alongthe FBG.

The period of the refractive index modulation may be linearly ornonlinearly chirped.

In some implementations, the refractive index modulation may have anapodized amplitude.

In accordance with yet another aspect, there is provided an opticaldevice supporting a light beam. The optical device includes an opticalfiber path having a core and at least one cladding, and configured toguide the light beam in a core mode along a propagation direction. Theoptical device further includes a first and a second Fiber Bragg Grating(FBG) successively disposed along the optical fiber path and each havingan input end and an output end with respect to said propagationdirection, each of said FBG comprising a refractive index modulation inthe core of the optical fiber path having a chirped period larger at theinput end than at the output end. The refractive index modulation isconfigured to allow a useful component of the light beam through theFBG. The refractive index of modulation of the first and second FBGs arefurther configured to couple a first and a second deleterious componentof the light beam into one or more counter propagating cladding mode ofthe optical fiber path, respectively, the first and the seconddeleterious components covering different spectral ranges.

Embodiments of the filters and optical devices described herein maycircumvent the disadvantages of the prior art as they rely on a gratingwritten in an optical fiber. Current design and fabrication techniquesprovide much flexibility in tailoring the spectral response of a fiberBragg grating (FBG). Furthermore, FBGs can be fabricated by submittingan optical fiber to an interference fringe pattern of UV radiation aslong as the fiber is photosensitive. Most optical fibers arephotosensitive at least in the core, where germanium is present toincrease the index of refraction. If needed, various sensitizationtechniques are available to increase the photosensitivity of the fiberto UV radiation. Moreover, a femtosecond laser in the near infrared canbe used to produce a grating in a fiber that is not photosensitive. Thepresent optical filter can thus be produced in a fiber already used in asystem or in less exotic fibers than those proposed for filtering.

In several fields of application, in-fiber filter of widebanddeleterious light such as described herein may be highly useful. Such afilter preferably attenuates deleterious light as strongly as possible.The attenuation spectrum of the filter preferably covers the spectrum ofthe deleterious light. Moreover, the attenuation of deleterious lightpreferably results from the deleterious light being taken out of thefiber core rather than reflected within the fiber core, since itsharmful effect usually does not depend on its direction of propagation.For example, ASE propagating in both directions can deplete the gaininside of a fiber laser. Likewise, Raman light propagating in eitherdirection can grow at the expense of a useful light signal. Therequirement of no reflection within the fiber core may be more criticalwhen a high gain is available for amplification of the deleteriouslight. In this case, minute reflections can provide sufficient feedbackto allow parasitic oscillations. A high Raman gain results when theoptical intensity, i.e. the power per unit area, is high and whenpropagation takes place over sizable distances. These conditions areeasily met in an optical fiber because of the small transversal extentof the core modes and the low fiber attenuation allowing propagationover long distances. The Raman gain can be so high that Rayleighbackscattering in the fiber suffices to induce oscillations. Under suchconditions, the wideband filtering of deleterious light is preferablyperformed while minimizing its reflection in the fiber core.

In some implementations, the filter advantageously has as little impactas possible on useful light propagating in the fiber. Attenuation ofuseful light by coupling to cladding modes is preferably minimized oravoided. Reflection of useful light within the fiber core is preferablyminimized or avoided as well. In applications where light propagating inthe fiber is on its way towards a point of use, a reflection into thefiber core or cladding may represent a detrimental loss. The lack ofreflection of useful light in the fiber core may be of particularusefulness when the filtering takes place inside a fiber laser cavity.When an optical component is inserted inside a laser cavity, care mustbe exercised to avoid that parasitic reflections on this component getcoupled to the laser modes. Whenever this happens, coupled cavities arecreated that renders the laser behaviour susceptible to interferentialperturbations. An unstable output ensues. In a bulk laser, the properalignment of an inserted optical component can ensure that reflectionson the component exit the laser cavity without coupling to laser modes.In a fiber laser, the filter should minimize the reflection of light inthe fiber core, since modes guided by the fiber core are generally thoseparticipating in the oscillation of a fiber laser.

Other features and advantages of the invention will be better understoodupon a reading of embodiments thereof with reference to the appendeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematised representation of a FBG provided in an opticalfiber path.

FIG. 2a is a graph of the transmittance of a simulated strong FBGwritten in a SMF28 fiber in which only the core is photosensitive with aBragg wavelength of 1.576 μm; FIG. 2b is the reflectance of the samesimulated FBG.

FIG. 3 (PRIOR ART) shows a configuration using a uniform period FBGcoupled to a circulator to separate useful light from widebanddeleterious light according to prior art.

FIG. 4 is a schematized representation of a CFBG.

FIG. 5a (PRIOR ART) is a schematized representation of the use of a CFBGto filter light, according to prior art; FIG. 5b illustrates a use of aCFBG avoided in the prior art.

FIG. 6 is a schematized representation of a CSFBG.

FIGS. 7a and 7b present the transmittance calculated with the samegrating as in FIG. 2, but with a small tilt angle of the grating fringesequal to 2.6 degrees (FIG. 7a ) or 5.2 degrees (FIG. 7b ) respectively.

FIG. 8 is a partial cross-sectional side elevation view of a tuningmechanism applying a temperature gradient on a CSFBG according to oneimplementation.

FIG. 9a is a schematized representation of the use of a CSFBG in anoptical fiber path with light entering from the red side; FIG. 9b is aschematized representation of the use of similar CSFBG with lightentering from the blue side.

FIG. 10a shows the reflectance of a simulated CSFBG used in accordancewith the configurations shown in FIG. 9a . (dotted lines) and FIG. 9b(solid lines). FIG. 10b shows the transmittance of a simulated CSFBG,that is similar for the configurations of FIG. 9a and FIG. 9 b.

FIG. 11 schematically illustrates the expected shift to shorterwavelengths of the contribution of a higher order core mode to thespectral response of a CSFBG written in a multimode fiber.

FIGS. 12a and 12b illustrate the variation of the period of opticalfilters respectively based on a linearly chirped and a nonlinerarlychirped FBG.

FIGS. 13a and 13b schematically illustrate the use of a filter with afiber laser cavity according to two variants.

FIG. 14 schematically illustrates the use of a cascade of unidirectionalfilters with different periods according to one embodiment.

FIG. 15a is a schematized representation of a bidirectional filter wherethe period of the fringes is shortest at some point within the filterand increases towards both ends of the filter; FIGS. 15b and 15crespectively show the grating period variation of such a filter based onlinearly chirped and nonlinearly chirped segments.

FIG. 16a is a schematized representation of a bidirectional filter usingcascaded unidirectional filters; FIGS. 16b and 16c respectively show thegrating period variation of such a filter based on linearly chirped andnonlinearly chirped unidirectional filters.

FIG. 17 is a graph of the measured spectral response of bidirectionalfilter.

FIGS. 18a to 18c schematically represents various configurations makinguse of filters according to some embodiments.

FIG. 19a is a representation of the range of wavelengths at whichreflection in the core mode occurs at the input end of a filteraccording to an embodiment; FIG. 19b presents the spectral domain overwhich light is reflected by fringes at the output end of the samefilter; and FIG. 19c shows the global spectral domain obtained from thecombination of the spectral responses from all positions along thefilter.

FIG. 20 is a schematized representation of a CFBG having grating fringescovering a portion only of the core.

FIG. 21 is a schematized representation of a CFBG having grating fringeswith a tilt angle that varies along the grating.

DETAILED DESCRIPTION

The present description is concerned with optical devices and opticalfilters providing the wideband filtering of deleterious lightpropagating in an optical fiber.

Implementations of the filters described above may be used in a varietyof contexts. The optical device may be embodied by any one of severaldevices which includes one or more optical fibers which support thepropagation of a light beam having a useful component and a deleteriouscomponent. As one skilled in the art will readily understand, theexpression “useful light component” is meant to encompass any lightgenerated and/or guided in the optical fiber which is intended for use,and/or which is to propagate within the optical fiber unhindered. Theexpression “deleterious light component” is meant to refer to lighthaving properties which may damage the optical device or other elementsin a path of the light or more generally to light which one wishes toremove from the optical device.

It will be further understood that the expression “light component” ismeant to refer to a portion of a light beam at a particular wavelength,within a particular spectral range or having a particular spectralprofile. The useful and deleterious light components can therefore beunderstood as groups of photons of different wavelengths within a samelight beam.

The optical devices incorporating filters such as described herein mayfor example be useful in fiber lasers and fiber laser systems where ASEor Raman scattering is problematic. The filter can be used inside thefiber laser itself or outside, for example in amplification stages or ina delivery fiber. In one implementation, the filter may be useddownstream the laser cavity of a Master Oscillator Power Amplifier(MOPA), for example in an amplification stage. In applications requiringthe transport of a sizable optical power over long optical fibers, aperiodical distribution of filters along the fiber can avoid reachingthe threshold level at which Raman scattering becomes sizable. It willbe further understood that the contexts listed herein are provided byway of example only and the embodiments of the invention describedherein may be useful in other applications.

In some implementations, the optical device includes an optical fiberpath (or optical fiber) which includes a core and a cladding. In someembodiments, the optical fiber may be a multi-clad fiber, that is, havea plurality of claddings. The optical fiber is configured to guide thelight beam in a core mode along a propagation direction. The concepts ofmodes and propagation directions are elaborated on further below. Insome implementations, the optical fiber may be a PolarizationMaintaining (PM) fiber.

The optical device further includes a Fiber Bragg Grating (FBG) disposedalong the optical fiber path. The FBG is also referred to as a filter oroptical filter or deleterious light filter in the present description.The FBG has an input end and an output end with respect to thepropagation direction of the light beam in the core of the opticalfiber. The FBG is or includes a refractive index modulation in the coreof the optical fiber path defining slanted grating fringes with achirped period. As will be explained further below, the period of therefractive index modulation is advantageously larger at the input endthan at the output end of the FBG. Furthermore, the refractive indexmodulation is designed to allow the useful component of the light beamthrough the FBG and to couple the deleterious component of the lightbeam into counter propagating cladding modes of the optical fiber path.In other words, the filtering of the deleterious component of the lightbeam out of the optical fiber is realized without significantlyaffecting the useful light component.

In typical implementations, the filter takes deleterious light coveringa wide spectral band out of the fiber core without reflecting this lightinto the fiber core. It also allows the unhindered transmission ofuseful light at a wavelength outside of the spectral band covered by thedeleterious light. More specifically, incoming deleterious light iscarried by core modes of the fiber. The filter couples the incomingdeleterious light to cladding modes propagating in the oppositedirection without coupling the incoming deleterious light to core modespropagating in the opposite direction. Likewise, incoming useful lightis carried by core modes. The filter transmits the useful light withoutcoupling it to either core modes or cladding modes propagating in theopposite direction.

To more clearly understand the characteristics of the present filter,the following sections expose theoretical considerations useful to theunderstanding of the physics behind the operation of the optical deviceand its components.

Optical fiber In its most simple form, an optical fiber comprises a corewith an index of refraction n_(co), embedded in a cladding with an indexof refraction n_(cl) that is lower than the core index, i.e.

n _(cl) <n _(co)  (1)

Both the core and cladding are made of a glassy material with a lowattenuation at optical wavelengths. The cladding is covered with apolymer coating.

The optical field of light traveling in an optical fiber can bedecomposed as a superposition of modes, a mode being a transversal fielddistribution that maintains its shape as it propagates along the fiber.Assuming the fiber is aligned with axis z, the optical field of a modepropagating along the fiber varies according to:

$\begin{matrix}{\exp\left( {{\pm j}\frac{2\pi}{\lambda}n_{eff}z} \right)} & (2)\end{matrix}$

where the sign depends on the direction of propagation, λ is the vacuumwavelength of the light guided by the fiber and n_(eff) is the effectiveindex of the mode. The modal field distribution and the effective indexvary from mode to mode. Moreover, they are dependent on the transversalstructure of the optical fiber and on the wavelength. According toequation (2), a mode can be expressed as a superposition of plane wavesthat all have the same longitudinal wavenumber k_(z) equal to

$\begin{matrix}{k_{z} = {{\pm \frac{2\pi}{\lambda}}n_{eff}}} & (3)\end{matrix}$

where the sign depends on the direction of propagation. One candistinguish between core modes and cladding modes. Core modes arecharacterized by an effective index larger than the cladding refractiveindex, i.e.

n _(cl) <n _(eff) <n _(co)  (4)

The transversal wavenumber k_(t) of these modes is then imaginary in thecladding since

$\begin{matrix}{k_{t} = {\frac{2\pi}{\lambda}\sqrt{n_{cl}^{2} - n_{eff}^{2}}}} & (5)\end{matrix}$

As a result, a core mode decreases exponentially within the cladding andis confined to and near the fiber core. As a result, core modes do notinteract with the polymer coating covering the cladding. On the otherhand, the effective index of a cladding mode is smaller than thecladding refractive index, i.e.

n _(eff) <n _(cl).  (6)

The transversal wavenumber of a cladding mode remains real in thecladding. As a result, a cladding mode remains oscillatory in thecladding where it can carry a sizable fraction of its power. Contrary tocore modes, cladding modes interact with the polymer coating.

The number of core modes guided by an optical fiber increases with thediameter of the core and the difference between the index of refractionof the core and that of the cladding. A single mode fiber is thelimiting case of a fiber that carries only one core mode. Whenpolarization is considered, a single mode fiber actually guides two coremodes that are orthogonally polarized to one another. In practice,optical fibers have a cladding outer diameter that is much larger thanoptical wavelengths. As a result, a very large number of cladding modesare supported by the fiber. These modes interact with the polymercoating and are typically attenuated over much shorter distances thancore modes. Most applications involving an optical fiber rely ultimatelyon core modes that are guided with little attenuation.

It is known in the art that optical fibers may differ in practice fromthis simple description. For example, the index of refraction of thecore may vary radially as in graded index fibers. An optical fiber mayhave multiple claddings to guide pump light used to create an inversionof population in the fiber core containing an active species.Notwithstanding these details, the fact remains that optical fibersgenerally guide core modes that are confined into and near the core ofthe optical fiber and cladding modes that extend well outside of thecore.

Fiber Bragg Gratings

Fiber Bragg gratings (FBGs) are convenient components for performingvarious spectral functions inside an optical fiber. They are usedextensively in the fields of telecommunications, sensing and fiberlasers [R. Kashyap, Fiber Bragg Gratings, Academic Press, 1999; and A.Othonos and K. Kalli, Fiber Bragg Gratings. Fundamentals andApplications in Telecommunications and Sensing, Artech House, 1999]. AFBG generally consists in a periodic or quasi periodic modulation of theindex of refraction (or refractive index) extending along a segment ofan optical fiber (see FIG. 1). This modulation can produce the transferof optical power from one mode to another inside an optical fiber: themodulation is said to couple two modes together. Two conditions must bemet for this transfer of power to take place.

Phase Matching

Each period of a grating scatters a weak optical field from a first modeto a second mode. To achieve a sizable transfer of power from one modeto another, all these weak scattered fields must add up in phase. Thisfirst condition, termed phase matching, is a manifestation of thecollective nature of the mode coupling: all grating periods mustcontribute in phase to provide an efficient mode coupling. Thisintuitive point of view is supported by an electromagnetic analysis ofthe situation. Mathematically, the phase matching condition between twomodes propagating in opposite directions may be expressed as

$\begin{matrix}{{\frac{2\pi}{\lambda}\left( {n_{{eff}\; 1} + n_{{eff}\; 2}} \right)} = \frac{2\pi}{\Lambda}} & (7)\end{matrix}$

where λ is the wavelength of the light, ∧ is the period of the indexmodulation, whereas n_(eff1) and n_(eff2) are the effective indexes ofthe modes being coupled to one another. This means that a grating withperiod ∧ can transfer power from a first mode with effective indexn_(eff1) to a second mode with effective index n_(eff2) propagating inthe opposite direction when the first mode carries light at wavelength λverifying equation (7). The transfer of power will be much lessefficient or even negligible if light is at another wavelength.Likewise, the phase matching condition between two modes propagating inthe same direction is met when

$\begin{matrix}{\left. \frac{2\pi}{\lambda} \middle| {n_{{eff}\; 1} - n_{{eff}\; 2}} \right| = \frac{2\pi}{\Lambda}} & (8)\end{matrix}$

According to these equations, longer modulation periods are required tocouple two modes propagating in the same direction. A grating designedto couple co-propagating modes is called a long period grating (LPG). Ina single mode fiber, a LPG is used to couple the fundamental mode toco-propagating cladding modes. The spectral response of a LPG is quitesusceptible to various perturbations, such as a temperature variation,because the phase matching condition depends on the difference betweenthe effective indexes of the modes being coupled. This difference canvary sizably, in relative terms, in the presence of a temperaturevariation, especially when coupling takes place between a core mode anda cladding mode that have markedly different transversal profiles.

The appellation Fiber Bragg Grating (FBG) is typically reserved in theart to gratings that couple modes propagating in opposite directions.The phase matching condition in a FBG depends on the summation of theeffective indexes and the spectral response is more stable in thepresence of perturbations. In considering the coupling betweencounter-propagating modes, only equation (7) needs to be considered inthe description below. According to equation (7), the phase matchingcondition between two given modes is verified at a specific wavelengththat depends on the modulation period and on the effective index of themodes, i.e.

λ=(n _(eff1) +n _(eff2))∧  (9)

The phase matching condition thus leads to a longitudinal requirement:for the grating to couple two given modes at wavelength λ, the period ofthe grating must verify equation (9). In general, the effective index ofeach mode depends on the wavelength. This complicates the evaluation ofthe wavelength at which the coupling between two given modes takes placefor a given grating period. Likewise, this must be taken into accountwhen determining the grating period required for coupling two givenmodes at a target wavelength.

The wavelength at which phase matching is realized depends on theeffective indexes of the modes being coupled. In the case of a gratingpresent in an optical fiber supporting many modes, each mode having aspecific effective index, the phase matching can be realized at multiplewavelengths, each wavelength being associated to a given pair of modes.The fundamental mode of an optical fiber has the largest effective indexn_(eff0). The longest wavelength at which phase matching is realized isthus equal to

λ_(B)=2n _(eff0)∧  (10)

Wavelength λ_(B), at which the fundamental core mode of a fiber isreflected on itself by a grating of period ∧, is called the Braggwavelength. This appellation stems from the fact that, historically,FBGs have been used mostly to reflect the core mode of a single modefiber at wavelength λ_(B).

Scattering Efficiency

The phase matching condition determines the wavelength at which agrating of a given period can couple a first mode to a second mode.Whether this coupling actually takes place depends on a secondcondition, i.e. the scattering efficiency from the first mode to thesecond mode. Even though the phase matching condition is met, notransfer of power takes place if each individual grating period scattersno power from the first mode to the second mode. To illustrate thispoint further, let us consider a FBG with index modulation fringes thatare normal to the axis of the optical fiber, as shown in FIG. 1. It isfurther assumed that the spatial extent of the index modulation normalto the optical fiber axis is sufficient to completely intercept a givenmode. Since the phase front of a mode in an optical fiber is a planenormal to the fiber axis, it is understood that a grating as justdescribed can only reflect the mode on itself. In other words, underthese conditions, coupling can only occur between an incident mode andthe same mode propagating in the opposite direction.

As a next step, it is assumed that the index modulation does notcompletely intercept an incoming mode. In this case, each grating periodacts as a finite area mirror that reflects only part of an incomingbeam. Reflection of an incoming field by a finite area mirror produces areflected field with a plane wave angular spectrum different from thatof the incoming field, more so if the transversal area of the mirror issmall relatively to the transversal area of the incoming field. In anoptical fiber, reflection by a grating with a transversal extent smallerthan that of an incoming mode is thus expected to scatter some lighttowards other modes.

This situation is encountered in practice in single mode fibers in whichthe FBG is limited to the fiber core. Since a core mode extends somewhatoutside of the core, scattering towards other modes is expected. In asingle mode fiber carrying only one core mode (excluding polarizationconsiderations), optical power gets scattered towards cladding modes.When the fundamental core mode in such a fiber is incident on a grating,one observes in transmission a dip at the Bragg wavelength 40 predictedby equation (10) and a series of dips at shorter wavelengths resultingfrom the coupling to various cladding modes with smaller effectiveindex, as shown in FIG. 2a . This graph was obtained by simulating astrong FBG written in a SMF28 fiber in which only the core isphotosensitive. The Bragg wavelength of the grating was 1.576 μm. Thereis a narrow spectral gap 44 between the Bragg wavelength and thewavelength at which coupling to a first cladding mode occurs. This gapis wider in an optical fiber with a large numerical aperture and istypically between 1 and 5 nanometer wide, but could be as small as 0.1nanometer or smaller. In reflection, a single peak 46 is observed at theBragg wavelength because cladding modes to which optical power has beencoupled by the grating are typically attenuated prior to detection (seeFIG. 2b ). In most applications where reflection of the core mode in thefiber core is of interest, such loss of power to cladding modes isconsidered a nuisance. This is the case, for example, when a FBG is usedto reflect a communications channel at wavelength λ_(B) and transmitunhindered channels located at other wavelengths. Coupling to claddingmodes can then induce losses to channels located at wavelengths shorterthan λ_(B). Indeed, to avoid such coupling to cladding modes, opticalfibers have been developed with a photosensitive area extending outsideof the fiber core in order for the grating to intercept the entire coremode [J. M. Kim et al., Suppression of cladding-mode coupling loss infiber Bragg gratings by independent control of refractive index andphotosensitive profiles in a single-mode optical fiber, IEEE PhotonicsTechnology Letters 12, 1504-1506 (2000); and V. Bhatia et al., FiberBragg grating with cladding mode suppression, U.S. Pat. No. 6,351,588].

Mathematically, the scattering efficiency between two modes iscalculated with a superposition integral involving the transversalprofiles of the FBG index modulation and of the modal fields. Thisintegral must usually be evaluated numerically, but symmetry argumentscan be invoked to determine which family of modes can be coupled to oneanother. For example, a cylindrically symmetric grating written in acylindrically symmetric optical fiber can only couple modes that havethe same azimuthal variation. A cylindrical asymmetry in the indexmodulation, resulting for example from the side-writing procedure usedto inscribe the grating in the optical fiber, can lead to couplingbetween modes of different azimuthal orders. Finally, a stronger indexmodulation translates into a stronger scattering efficiency. Theamplitude of the refractive index modulation can be adjusted todetermine the level of power transfer between coupled modes.

Chirped Gratings

The coupling between two modes with effective index n_(eff1) andn_(eff2) is most effective at a wavelength defined by equation (9). Thespectral bandwidth over which this coupling remains significant dependson the length of the grating and on the amplitude of the indexmodulation. A long grating with a weak index modulation couples twomodes over a narrow wavelength range. The coupling spectral bandwidth isincreased by shortening the grating and increasing the amplitude of theindex modulation. At 1550 nm, this typically allows bandwidths on theorder of a couple of nanometers.

A preferred way of increasing the coupling bandwidth is by varying themodulation period along the grating. The grating is then said to bechirped and is identified in the following as a Chirped Fiber BraggGrating or CFBG (see FIG. 4). The wavelength of maximum coupling betweentwo modes then varies along the grating and the coupling spectralbandwidth of the whole grating can be determined mostly by the periodvariation along the grating. Bandwidths of many tens of nanometers canbe obtained at 1000 nm and 1550 nm. Moreover, the position at whichlight is reflected by a CFBG depends on the wavelength. In a single modefiber supporting a single core mode, a CFBG with a monotonous periodvariation provides an unambiguous group delay as a function ofwavelength, which can be used to compensate for the dispersion of anoptical fiber link. Dispersion compensation in optical fibercommunications links is a major application of CFBGs.

In most applications, a CFBG is used to reflect light into the fibercore. The coupling to cladding modes requires special consideration. Tosimplify the discussion, we consider a CFBG in a single mode fiber.Equation (10) is rewritten as

λ_(B)(z)=2n _(eff0)∧(z)  (11)

to highlight the variation of period ∧ as a function of position z alongthe grating, leading to a local Bragg wavelength λ_(B)(z) that dependson position z as well. At position z, a CFBG can reflect light at thelocal Bragg wavelength λ_(B)(z) in the core mode, but it can also couplelight at wavelengths shorter than λ_(B)(z) to cladding modes. To avoidlosses to cladding modes, a CFBG is typically used with light incidentfrom the side where the grating period is the shortest. Light thenpropagates freely into the FBG until it encounters the grating periodwhere condition (11) is met, at which point it gets reflected into thefiber core (see FIG. 5a ). Past that point, the small fraction of lightthat has not been reflected into the fiber core can get coupled tocladding modes. In the opposite case, light encounters first longerperiods that can couple it to cladding modes (see FIG. 5b ). Some of thelight is thus lost before it gets reflected into the fiber core. Theincurred loss is generally larger at shorter wavelengths that musttravel farther along the CFBG before being reflected into the fibercore. In the usual jargon, it is preferable to enter a CFBG from theblue side 50 (see FIG. 5a ) rather than from the red side 52 (see FIG.5b ) to avoid losses to cladding modes [M. Durkin et al., Equalisationof spectral non-uniformities in broad-band chirped fibre gratings, inProc. OSA Technical Digest BGPP '97, vol. 17, paper BMG16.1-3, 231-233(1997)].

Slanted Gratings

As aforementioned, an asymmetry in the transversal profile of therefractive index modulation allows the coupling of modes with differentazimuthal symmetries. Such asymmetry can be introduced on purpose bytilting the fringes of the index modulation. A FBG with index modulationfringes that are not perpendicular to the axis of the fiber, asillustrated in FIG. 6, is often called a slanted FBG and is identifiedas a SFBG in the following. Tilting the fringes favors the coupling ofcore modes to cladding modes. The transmission spectrum of a SFBG in asingle mode fiber is reminiscent of that of a strong grating thatintercepts only a portion of an incoming core mode; it presents a seriesof dips 42 at wavelengths shorter than the Bragg wavelength defined inequation (10). FIGS. 7a and 7b present the transmittance calculated withthe same grating as in FIG. 2, but with a small tilt angle of thegrating fringes equal to 2.6 degrees (FIG. 7a ) or 5.2 degrees (FIG. 7b). By comparing with FIG. 2, it can be seen that tilting the gratingfringes favours the efficient coupling of the fundamental core mode tomore numerous cladding modes. A slight spectral gap remains between theBragg wavelength and the wavelength at which coupling to cladding modesbegins.

The highly structured transmittance of a SFBG is not always desirableand can be smoothed by chirping the SFBG [I. Riant et al., U.S. Pat. No.6,321,008, (supra)]. In the following, a chirped and slanted FBG isnoted as CSFBG (as shown in FIG. 6). In a CSFBG, each segment of thegrating with a relatively uniform period produces a highly structuredtransmittance. However, the highly structured transmittances fromsegments with different periods are shifted spectrally from one another.The combination of these spectrally shifted transmittances results in aglobal transmittance that is wideband and smooth, as shown in FIG. 10 b.

A CSFBG can thus provide wideband optical attenuation, for example overmore than 10 nanometers, by coupling light from the fiber core to thecladding. As explained above, in a fiber with a single cladding,cladding modes interact with the polymer coating covering the fiber andare typically attenuated over much shorter distances than core modes.When the optical powers involved are moderate, this may suffice todissipate the power coupled to the cladding modes by the CSFBG. In highpower fiber lasers, measures may be needed to get rid of the opticalpower coupled into the cladding in a more controlled manner. This may beachieved with a segment of optical fiber covered by a polymer coatingwith an index of refraction that is higher than that of the fibercladding. In this case, the optical power propagating inside thecladding rapidly escapes into the coating, which is in thermal contactwith a heat sink to dissipate the heat generated by absorption of theoptical power. In fibers with multiple claddings designed to favor theguidance of pump light, the situation can get more involved. However,the general idea is the same, i.e. provide means allowing the escape oflight from the claddings into an attenuating medium that is in thermalcontact with a heat sink. Such an arrangement, known in the art as alight stripper, is already used to get rid of residual pump power [A.Wetter et al., High power cladding light strippers, Proc. SPIE 6873,687327 (2008)].

In some implementations, the filter used to attenuate deleterious lightor even suppress the generation of deleterious light preferably reflectsno light inside the fiber core (deleterious or useful). Contrary toknown prior art approaches, devices using filters as described herein donot require the use of a special fiber to achieve an ultralowreflectivity into the fiber core. The reduction in reflectivity is notachieved by optimizing the scattering efficiency through a proper designof the transversal structure of the optical fiber. It is achieved bytailoring the longitudinal variation of the phase matching conditionalong the grating, ensuring that incoming core modes are coupled tocladding modes before being reflected in the fiber core. Advantageously,ultralow reflectivity can be achieved at moderate tilt angles, i.e.without compromising the ability to define the filter spectral response.In some variants, the moderate tilt angle may be between about 1.5 and15 degrees. In some embodiments, the tilt angle may be about 2.6degrees, about 5.2 degrees or any value in-between. This has the furtheradvantage of minimizing the dependence of the spectral response on thepolarization. Ultralow reflectivity may also be achieved with stronggratings capable of providing a sizable attenuation of deleteriouslight. In some variants, such as for example shown in FIG. 21, the tiltangle of the grating fringes varies along the FBG.

Active Tuning

Various embodiments of optical filters described herein may be combinedwith or involve an active tuning mechanism coupled to one or more FBGs.Tuning of the grating may be used to fine tune the properties of the FBGto obtain a desired local average effective refractive index or Braggwavelength.

As known in the art, the wavelength of peak reflection for a Bragggrating can be shifted by a change in either the strain or thetemperature (or both) imposed on the grating. If the optical fibersegment hosting the Bragg grating is subject to a strain or temperaturegradient, the modulation period of the index of refraction pattern andthe mean index of refraction can be modified with the goal offine-tuning the dispersion characteristic of the grating. The tuningmechanism may therefore include an assembly changing the strain appliedto the optical fiber segment hosting the FBG, an assembly applying atemperature gradient to this optical fiber segment, or a combination ofboth.

In some implementations, the tuning mechanism may be configured to applya strain or temperature variation which is non-uniform along thegrating, that is, locally changing the temperature or strain alongdifferent portions of the grating. As will be readily understood by oneskilled in the art, a non-uniform heating or strain induces a chirp inthe grating, or modifies a pre-existing chirp. Controlling the magnitudeof the thermal gradient or strain variation controls the magnitude ofthe resulting chirp, and thus there is provided a form of localadjustment of the spectral reflectivity of the grating.

Referring to FIG. 8, there is shown an example of a tuning mechanism 70imposing a thermal gradient on a FBG embodying an optical filter 20. Inthis variant, the optical fiber 22 hosting the FBG is preferably inclose contact or proximity with an elongated heat conductive membercalled herein the natural gradient tube 80, inside which the fiber 22rests freely. The natural gradient tube 80 may have a cylindrical hollowshape and is preferably made of a good heat conductor, typically ametal. The natural gradient tube 80 allows a uniform heat transfer alongits length and thus creates a smooth temperature profile along thefiber. Advantageously, the natural gradient tube 80 can isolate thefiber 22 from surrounding temperature perturbations.

In some variants, a thermal compound may be provided between the naturalgradient tube 80 and the host optical fiber 22 to ensure a goodreplication of the temperature profile along the natural gradient tubein the fiber. In such an embodiment, the optical properties of the FBGare advantageously unaffected by the contact between the optical fiberand the natural gradient tube, and long-term reliability is promoted asno mechanical stress is applied to the optical fiber at any point.Within this preferred embodiment, the fiber can remain unaffected by thethermal expansion (or contraction) of the metallic tube, since they arenot mechanically coupled to one another.

The natural gradient tube 80 may further be thermally isolated from itssurroundings to ensure the quality of the induced thermal profile. ADewar type thermos system, with an inner shield to improve radiationisolation, can be used for this purpose. A low emissivity construction,using for example a rod with a mirror finish surface, may be used tofurther improve the performance of the device.

Referring still to FIG. 8, in the illustrated variant the tuningmechanism 70 includes four heat pumping elements 82 a, 82 b, 82 c and 82d affixed in close physical contact to the natural gradient tube 80. Aswill be readily understood, a different number of heat pumping elementsmay be provided. The heat pumping elements 82 are distributed, evenly orunevenly, along the length of the natural gradient tube 80. The contactbetween the natural gradient tube 80 and each heat pumping element 82may be ensured using an appropriate technique such as pressure mountingwith a thermal compound, thermal gluing, soldering, or the like. In somevariants, the contact between the natural gradient tube 80 and the heatpumping elements 82 may be indirect, using thermal bridging components84. The heat pumping elements 82 may for example be embodied by Peltiereffect Thermo Electric Coolers, referred hereafter as TECs. The TECs areconfigured to pump heat from one side of their body to the other,thereby controlling the temperature of the natural gradient tube at thepoint of contact therebetween. The thermal conductivity of the naturalgradient tube 80 allows the heat distribution along the tube 80 tosettle into a smooth temperature profile between the fixed temperaturepoints provided by the heat pumping elements 82.

The tuning mechanism 70 may further include one or more temperaturesensors (not shown) placed in close proximity to the natural gradienttube 80. The temperature sensor may for example be embodied by athermistor or a resistance temperature detector (RTD). By way ofexample, a RTD may be provided in association with each heat pumpingelement 82 a, 82 b, 82 c and 82 d. Each temperature sensor is affixed inclose contact with the natural gradient tube 80 using an appropriatetechnique, for example using a thermally conductive epoxy. Signals fromthe temperature sensors are used as input to a servo control system (notshown) to precisely control, that is, fix and maintain, the temperatureprofile along the grating. Such means for temperature control are wellknown in the art, and typically include appropriate control electronicsand drivers such as TEC controllers with PID servo-control for optimumdynamic operation.

In some implementations, the TECs 82 are directly mounted on a heat sink78. The heat sink 78 may for example be embodied by a standarddissipative heat sink provided with fins, or more simply by a large heatdissipation plate. In other variants, the heat sink 78 may be embodiedby a metallic casing used for packaging the FBG, such as shown in thevariant of FIG. 8. In further variants, the TECs 82 may be mounted on athermally conductive metallic recirculation bar (not shown) to improvethe energy efficiency of the whole device. Such an assembly is forexample shown in Canadian patent applications no. 2,371,106 and2,383,807 (LACHANCE et al).

Of course, other configurations may be used to provide a tuningmechanism couple to the FBG. By way of example, in other variants totuning mechanism may involve the application or modification of a strainprofile along the optical fiber hosting the FBG.

Embodiments and Uses of Optical Filters

Referring to FIG. 9A, the configuration of an optical filter 20according to a first embodiment is schematically illustrated.

The optical filter 20 is provided in an optical fiber 22 having a core24 and a cladding 26, the filter 20 extending along the core 24 of thisfiber 22. The filter 20 consists in a CSFBG having the followingspecific property: the period of the refractive index modulation of theCSFBG is largest at the input end 28, where a light beam 30 enters thefilter, rather than at the output end 32 of the filter 20, which iscontrary to the preferred usage of a CFBG. In other words, light entersfrom the red side 52 instead of the blue side 50. This orientation ofthe CSFBG advantageously reduces the reflectivity within the core 24 ofthe fiber 22 while providing a strong coupling of deleterious light outof the fiber core 24.

Referring to the longitudinal axis z of the optical fiber 22, let z=0 bethe position of the input end 28 of the filter 20 where the period ismaximum and z=L be the position of the output end 32 of the filter 20where the period is minimum, L representing the length of the filter 20.Let light of wavelength λ_(i)=λ_(B)(z_(i)), where λ_(B)(z_(i)) is theBragg wavelength at position z=z_(i) along the filter (0<z_(i)<L), beincident on the filter at z=0. For the present discussion, the incominglight is assumed to be carried by the fundamental core mode in a singlemode fiber. The phase matching condition for the reflection of theincoming light into the counter-propagating fundamental core mode is metat z=z_(i). Until the incoming light reaches this position, itencounters modulation fringes with longer periods. As discussed earlier,from a phase matching point of view, grating fringes with periods longerthan ∧(z_(i)) can couple the incoming light to counter-propagatingcladding modes. Moreover, the fringes of the CSFBG are tilted to enhancethe scattering efficiency between the incoming fundamental core mode andcounter-propagating cladding modes. The incoming light of wavelengthλ_(i) is thus efficiently coupled out of the fiber core 24 and into thefiber cladding 26 prior to being reflected within the fiber core 24 atposition z_(i). Had the light been incident on the filter from theopposite end, it would have been reflected within the fiber core beforebeing coupled out of the fiber core by longer period fringes (see FIG.9b ). This explains that entering the filter from the end of maximumfringe period leads to a much-reduced reflection into the counterpropagating core mode. In a properly designed filter, this mechanism hasbeen found to provide a sizable attenuation by coupling into claddingmodes and simultaneously a very small reflection into the fiber core.This mechanism cannot operate for incoming light at a wavelengthcorresponding to the maximum Bragg wavelength λ_(B)(0) of the filtersince it is immediately reflected in the fiber core by the front end ofthe grating. This limitation can be countered by ensuring that themaximum period of the filter translates into a maximum Bragg wavelengththat is longer than the maximum wavelength of the deleterious light. Asa result, all wavelengths of the deleterious light are coupled out ofthe fiber core before being reflected into the fiber core and thereflectivity into the core of the filter remains low at all wavelengthsthat need to be filtered.

The operation of an embodiment of the filter is further illustrated inFIGS. 19a to 19c . FIG. 19a displays the grating fringes and thespectral domain 54 over which light is reflected by the grating fringesat the input end of the filter. The vertical arrow represents the Braggwavelength 2n_(eff0)∧(0) at which these fringes reflect the fundamentalcore mode unto itself. As aforementioned, this is the largest wavelengththat can be reflected by these fringes as determined by phase matching.Incoming light in the fundamental core mode can also be reflectedtowards cladding modes at wavelengths shorter than the Bragg wavelength.The range of wavelengths at which such reflection occurs at the inputend of the filter is represented by the rectangle with vertical lines onFIG. 19a . The slight gap 44 between the Bragg wavelength and thewavelength domain at which reflection into cladding modes occurs is alsovisible in FIG. 7. According to equation (9), light at a shorterwavelength can be reflected into a cladding mode with a smallereffective index. The leftmost part of the rectangle is thus associatedto the reflection of light in cladding modes with a smaller effectiveindex. According to equation (5), cladding modes with a smallereffective index have a larger transversal wavenumber and can bedecomposed as a superposition of plane waves that propagate at a largerangle with regards to the fiber axis. The leftmost part of the rectangleis thus associated to the reflection of light into cladding modes thatpropagate at a larger angle with regards to the fiber axis. A moderatetilt angle of the CSFBG fringes can be chosen to keep at a low value thescattering efficiency between the fundamental core mode and suchcladding modes propagating at larger angles with regards to the fiberaxis. This appears clearly in FIG. 7, where transmittance dips resultingfrom coupling to cladding modes are much weaker at shorter wavelengthswhen a smaller tilt angle is used (FIG. 7a ). The width of the rectanglein FIG. 19a can thus be adjusted by a proper choice of the fringe tiltangle.

A spectral graph as shown in FIG. 19a can be associated to each gratingperiod along the filter. As the fringe period gets smaller from theinput end to the output end of the filter, the associated spectralresponse shifts towards shorter wavelengths. FIG. 19b presents thespectral domain 56 over which light is reflected by fringes at theoutput end of the filter, at which point the fringe period is minimum.The Bragg wavelength is now reduced to 2n_(eff0)∧(L) and the spectraldomain 56 over which the fundamental core mode gets reflected intocladding modes, illustrated by the rectangle with horizontal lines, isshifted towards shorter wavelengths as well.

The combination of the spectral responses from all positions along thefilter leads to a global spectral domain as illustrated in FIG. 19c . Asshown, the spectral domain 58 over which the fundamental core mode getsreflected towards cladding modes by the whole filter, illustrated by therectangle with both vertical and horizontal lines, should cover at leastthe spectrum of deleterious light, thus ensuring that it all getsfiltered. According to the above discussion, the width of this spectraldomain depends on the fringe tilt angle and the fringe period variationalong the grating. A larger tilt angle increases the bandwidth of eachindividual spectral response, whereas a larger period variation producesspectral responses that are more spread apart. In general, it ispreferable to keep the tilt angle small. As aforementioned, a small tiltangle allows a better control on the shape of the spectral response andreduces the polarization dependence of the spectral response. Increasingthe bandwidth of the spectral domain over which the filter induceslosses by coupling to cladding modes is thus better achieved byincreasing the fringe period variation along the filter. This, in turn,can be achieved by increasing the rate of change of the fringe period orthe length of the chirped grating or both.

The two vertical arrows in FIG. 19c delimit the range of wavelengths atwhich the fundamental core mode can be reflected unto itself by thefilter. As indicated in the figure, most of these wavelengths can alsobe coupled to cladding modes. As explained above, entering the gratingfrom the red side ensures that coupling to cladding modes occurs beforelight reaches the point along the grating where it can be reflected intothe fiber core: the deleterious light is then attenuated before beingreflected in the fiber core. To avoid any reflection of deleteriouslight into the fiber core, the deleterious light wavelength shouldfurther be smaller than the Bragg wavelength at the input end of thegrating as shown in FIG. 19 c.

A properly designed filter should avoid attenuation of the forwardpropagating useful light, either by reflection into the fiber core or bycoupling to cladding modes. Reflection into the fiber core is avoidedwhen the useful light wavelength is either smaller than the minimumBragg wavelength 2n_(eff0)∧(L) or larger than the maximum Braggwavelength 2n_(eff0)∧(0). Graphically, this means that the useful lightwavelength is not between the two vertical arrows in FIG. 19c .Likewise, attenuation by coupling to cladding modes is avoided as longas the useful light wavelength is not within the spectral region overwhich such coupling can occur. Graphically, this means that the usefullight wavelength is not found within the rectangle in FIG. 19c . Toexplain how these requirements impact the design of the filter, twocases must be considered.

In a first situation, the useful light has a wavelength larger than thedeleterious light. In this case, it suffices that the period of theCSFBG at any position z_(i) be shorter than the period required forphase matching between counter-propagating fundamental core modes at thewavelength of the useful light. In other words, the Bragg wavelength2n_(eff0)∧(z_(i)) is shorter than the useful light wavelength at allpoints along the filter. Graphically, this means that the useful lightwavelength is at the right of the rightmost vertical arrow in FIG. 19c .As made clearly visible by FIG. 19c , this also ensures that noreflection of the useful light can occur into counter-propagatingcladding modes. In such a filter, the absence of any reflection of theuseful light thus rests on the lack of phase matching between thefundamental core mode and any other mode of the fiber at the usefullight wavelength.

In a second situation, the useful light has a wavelength shorter thanthe deleterious light. In this case the filter can be designed with aperiod that is at all points too long to provide phase matching forreflection of the useful light into the counter-propagating fundamentalcore mode. In other words, the filter is designed such that the Braggwavelength 2n_(eff0)∧(z_(i)) is larger than the useful light wavelengthat all points along the filter. This ensures that the useful light isnot reflected into the fiber core. Graphically, this means that theuseful light wavelength is at the left of the leftmost vertical arrow inFIG. 19c . FIG. 19c makes it clear that this condition may not always besufficient to ensure a proper operation of the filter, since the usefullight wavelength may still be found in the rectangle identifying thespectral domain over which coupling to cladding modes can take place. Toavoid attenuation of the useful light by coupling to cladding modes, itis also necessary that the spectral region over which the coupling tocladding modes occurs does not extend to a wavelength short enough thatit includes the wavelength of the useful light. As explained above, theshorter wavelength limit at which coupling to cladding modes can occurcan be adjusted by a proper choice of the grating period variation andof the fringe tilt angle. Graphically speaking, this means that thegrating period variation and the fringe tilt angle must be designed suchthat the rectangle in FIG. 19c does not extend down to the useful lightwavelength. In such a filter, the absence of any reflection of theuseful light thus rests on a lack of phase matching betweencounter-propagating fundamental core modes at the useful lightwavelength, but also on a reduced scattering efficiency between thefundamental core mode and cladding modes at the useful light wavelength.

FIGS. 10a and 10b present the simulation results of a CSFBG such as theone of FIG. 9a in a SMF28 fiber. The graphs present the reflectance andtransmittance of the fundamental core mode polarized either in the planeof incidence of the grating fringes (p-polarized, circles) orperpendicularly to the plane of incidence of the fringes (s-polarized,squares). The small tilt angle of the fringes (2.6 degrees) leads to avery slight dependence of the spectral response on polarization. Forexample, in the attenuation band of the filter where the transmittancegoes down to −12.5 dB, the polarization dependence is less than 0.3 dB.The transmittance does not depend on the side from which light entersthe CSFBG. However, the filter is clearly unidirectional as far asachieving a low reflectance is concerned. When light enters the CSFBGfrom the blue side (solid lines), the reflectance reaches over −4 dB,i.e. nearly 40% of the light is reflected by the grating. On the otherhand, when light enters the CSFBG from the red side (dotted lines), thereflectance within the attenuation band is smaller than −22 dB, i.e.less than 0.6% of the light is reflected within the fiber core. Alsovisible is the increase of the reflectance for light incident from thered side at wavelengths long enough for the fundamental mode to bereflected early in the grating before enduring losses to the claddingmodes.

As aforementioned, gratings written in an optical fiber with aphotosensitive area not covering the complete transversal profile of anincoming core mode can couple this mode to cladding modes, even in theabsence of any fringe tilt. This effect can take place in an opticalfiber where only the core is photosensitive. It can be further enhancedby grating fringes covering a portion only of the core, such asillustrated in FIG. 20. (Grating fringes covering a portion only of thecore can be produced for example with a femtosecond laser.) In such acase, the mechanism of operation of the inventive filter can take placewith grating fringes normal to the fiber axis. In principle, a CFBG canthus be used instead of a CSFBG in some implementations of the presentinvention. A CFBG may be advantageous as it is in general easier tofabricate than a CSFBG. In practice, however, a zero-tilt angle may notalways be optimal to ensure the best filter performance. The efficiencyof coupling to cladding modes and thus the performance of a filter builtwith a CFBG will depend on the transversal structure of the opticalfiber or the grating fabrication method.

In implementations using a CSFBG, the fringe tilt can be used to controlthe coupling to cladding modes. As seen in FIGS. 2 and 7, a fringe tiltincreases the number of modes to which a core mode can be coupledefficiently. This favors a rapid attenuation of an incoming core mode bylong period fringes before it reaches grating fringes with a periodleading to its reflection within the core of the fiber. A fringe tiltthus favours a strong attenuation and a reduced reflection within thefiber core. Furthermore, as seen in FIGS. 2 and 7, a fringe tilt canalso decrease the reflection at the Bragg wavelength. This means that ina CSFBG, a proper tilt angle can reduce the fraction of light that hasnot been lost to cladding modes that will actually get reflected in thefiber core. In some implementation of the disclosed filter, a CFBG maybe viewed as the special case of a CSFBG with a zero-tilt angle thatwill in general provide sub-optimal performances. Whether an easier tofabricate CFBG can provide sufficient performances will depend on theapplication at hand.

In the preceding paragraphs, the operation of the filter has beenexplained assuming that the incoming light is carried only by thefundamental core mode of a single mode fiber. In some embodiments, thefilter described herein may be used in a multimode fiber supporting morethan one core mode. Higher order core modes have effective indexessmaller than the fundamental core mode. Phase matching of a higher ordermode to a given cladding mode at a given grating period thus occurs at ashorter wavelength. Furthermore, the transversal profile of a higherorder core mode differs from that of the fundamental core mode. Hence,the scattering efficiency from higher order core modes to a givencladding mode may also differ from the scattering efficiency of thefundamental core mode to the same cladding mode. Notwithstanding this,the contribution of a higher order core mode to the spectral response ofa CSFBG written in a multimode fiber is expected to be generally shiftedtowards shorter wavelengths as shown in FIG. 11. A transition zone atshort wavelengths 60 thus appears where only higher order core modes arefiltered by the CSFBG. A CSFBG designed to attenuate all core modescarrying deleterious light and transmit unhindered all core modescarrying useful light located at a shorter wavelength than deleteriouslight can thus fully provide the desired attenuation of deleteriouslight only down to a wavelength slightly larger than the useful lightwavelength. Likewise, a transition zone exists 62 at long wavelengthswhere only lower order core modes are filtered by the CSFBG. A CSFBGdesigned to attenuate all core modes carrying deleterious light andtransmit unhindered all core modes carrying useful light located at alonger wavelength can thus fully provide the desired attenuation ofdeleterious light only up to a wavelength slightly shorter than theuseful light wavelength. These transition zones of incompleteattenuation of deleterious light will be narrower in an optical fiberwith a low numerical aperture, since the effective indexes of core modesare then closer to one another and the contributions to the spectralresponse of the various core modes will be more closely packed.Furthermore, similarly as in a single mode fiber, a large refractiveindex modulation and a very large number of cladding modes are helpfulin ensuring that all core modes are coupled effectively to claddingmodes. On the other hand, the large index modulation enhances the riskof having light reflected into the fiber core, hence the importance ofentering the grating from the longer period side.

Referring to FIGS. 12a and 12b , in some embodiments the period of thefilter 22 decreases monotonously from the input end 28 to the output end32. In some variants, such as shown in FIG. 12a , the rate of change ofthe period of the filter may be constant along the length of the FBG,thereby defining a linearly chirped grating. In other variants, such asshown in FIG. 12b , the rate of change of the period of the filter maybe variable along the length of the FBG, thereby defining a nonlinearlychirped grating. The range of periods, the tilt angle and the amplitudeof the refractive index modulation are chosen to ensure efficientcoupling of core modes to counter-propagating cladding modes over thespectral band covered by deleterious light, while avoiding such couplingat the wavelength of useful light. The design of the CSFBG can include avariation of the amplitude of the refractive index modulation(apodisation), of the chirp rate and of the tilt angle along the gratingfor added flexibility in tailoring the spectral response of the filter.Furthermore, as well known in the art (see e.g. J. Lauzon and F.Ouellette, Use of a temperature gradient to impose a chirp on a fibrebragg grating, U.S. Pat. No. 5,671,307; and L. E. Adams et al., Methodof making optical chirped grating with an intrinsically chirped gratingand external gradient, U.S. Pat. No. 6,169,831), the spectral responseof a chirped grating can be tuned by applying a temperature or straingradient along the grating. Such a gradient can be applied to the CSFBGto tune the bandwidth and amplitude of its spectral response.

The embodiment of FIGS. 9a and 12 is meant as a unidirectional filter tobe used with light incident from one side only, where the filter periodis largest. An example of application would be the filtering ofdeleterious light inside a fiber laser cavity, as shown in FIGS. 13a and13b , with the filter located at one end of the cavity in closeproximity to a FBG used as a mirror reflecting useful light but notdeleterious light. The end of the filter where the period is largestfaces the cavity. The filter can be on either side of the FBG mirror, ascontrasted in FIGS. 13a and 13 b.

Referring to FIG. 14, in some implementations, the optical filter or FBGmay be embodied by a cascade of unidirectional filters with differentperiods. Such a configuration can provide attenuation on differentspectral bands, located for example on both sides of the wavelength of auseful light. For example, one filter provides attenuation atwavelengths shorter than the useful light wavelength and another oneprovides attenuation at longer wavelengths. The unidirectional filtershave the same orientation with the ends where the period is maximumfacing in the same direction. This cascade is preferably used as aunidirectional filter. The arrows indicate the appropriate direction ofpropagation of light through such a cascade. It will be readilyunderstood that although only two filters are shown in the cascadeillustrated in FIG. 14, a higher number of such filters each covering adedicated portion of the spectrum may alternatively be provided.

As mentioned above, the tilt angle of the grating fringes can render thefilter spectral response dependent on polarization, more so if the tiltangle is large. In another variant (not shown), to reduce thisdependence on polarization, two unidirectional filters can be cascaded,with the second filter rotated by 90 degrees with regards to the firstfilter, the axis of rotation being the axis of the optical fiber. As aresult, light polarized in the plane of incidence of the first filterfringes is polarized perpendicularly to the plane of incidence of thesecond filter fringes and vice versa. The input ends of both directionalfilters face in the same direction as in the previous embodiment (seeFIG. 14) and this cascade should be used as a unidirectional filter.

In another implementation, the filter may have a period variation whichis not monotonous. Referring to FIG. 15a , there is illustrated oneexample of a filter 20 where the period of the fringes 34 is shortest atsome point within the filter and increases towards both ends of thefilter. This variant may in essence correspond to the combination of twounidirectional filters such as described above disposed back to backalong the fiber core 24 in opposite directions. This filter isbidirectional, ensuring a low reflectivity in the fiber core for lightincident at either ends of the filter. It could be used, for example,inside a fiber laser cavity at a position where deleterious light isexpected to be incident from both sides. The refractive index profile ofeither side of the filter 20 may be linearly chirped, such as shown inFIG. 15b , or nonlinearly chirped, such as shown in FIG. 15c . Theperiod profile of this embodiment may be symmetrical or not. As in aunidirectional filter, apodisation and longitudinal variation of thechirp rate and tilt angle can be used for more flexibility in shapingthe spectral response. Active tuning can be implemented as well byvarying the temperature or strain along the filter. Cascades ofbidirectional filters similar to those discussed above in the case ofunidirectional filters can also be used to provide attenuation indifferent spectral bands or reduce the polarization dependence of thespectral response.

In other implementations, a bidirectional filter can also be obtained bycascading two unidirectional filters with opposite orientations suchthat light incident on either side of the cascade encounters longerperiods first (see FIG. 16a ). The cascade can for example beimplemented by splicing together two fiber segments, each segmentcontaining one unidirectional filter. The refractive index profile ofeach unidirectional filter of the cascade may be linearly chirped, suchas shown in FIG. 16b , or nonlinearly chirped, such as shown in FIG. 16c. Both unidirectional filters may also be written at different positionsalong a common fiber segment. Such a cascade may thus be used with lightpropagating in both directions as shown by the arrows. The filters inthis cascade may be similar or dissimilar. Active tuning may be appliedto either one or both of the unidirectional filters. The distancebetween the filters is in principle arbitrary but in practice shouldpreferably be short enough that no significant amount of deleteriouslight is generated in-between. Such bidirectional filters can becascaded to provide filtering of different spectral bands or reduce thedependence to polarization. FIG. 17 presents the measured spectralresponse of a cascade of two unidirectional filters. The filter providesan attenuation of more than 28 dB over a bandwidth of 23 nm. The filteris clearly bidirectional, displaying a low reflectivity in bothdirections. The reflectivity within the attenuation band is actuallysmaller than that displayed in the graph, the −28 dB floor correspondingto the noise level of the measurement system in reflection. Thereflectance of light entering from the red side increases on the longwavelength side of the attenuation band.

The CSFBGs forming the cascade described in the previous embodiment canbe combined with other filters as well. For example, they can becombined with a FBG that partially reflects light at a usefulwavelength. The resulting cascade of three filters can be used, forexample, as the output coupler of a fiber laser wherein the usefulwavelength corresponds to the desired wavelength of emission of thefiber laser. The FBG provides feedback at the laser wavelength andallows some coupling out of the laser. A first CSFBG with longer periodsturned towards the laser cavity provides attenuation of deleteriouslight within the laser cavity while ensuring an ultra-low reflection ofthe deleterious light towards the laser cavity. A second CSFBG withlonger periods facing away from the laser cavity provides attenuation ofdeleterious light propagating towards the laser cavity from the outsidewhile ensuring an ultra-low reflection of the deleterious light awayfrom the laser cavity. In such an arrangement, the FBG can be locatedbetween the CSFBGs or on either side of the pair of CSFBGs, as shown inFIG. 18.

Any of the previous embodiments can be collocated with a light stripperto evacuate the optical power diverted to cladding modes, in which casethe filter is preferably mounted inside a package allowing heatdissipation.

Of course, numerous modifications could be made to the embodimentsdescribed above without departing from the scope of the invention.

1-16. (canceled)
 17. A fiber laser cavity, comprising: an optical fiberpath having a core and at least one cladding surrounding the core, theoptical fiber path having opposite extremities; a pair of cavity mirrorsdisposed at either extremity of the optical fiber path; and adeleterious light filter disposed in the optical fiber path between thecavity mirrors, the deleterious light filter including a Fiber BraggGrating (FBG) having an input end and an output end, the FBG having arefractive index modulation in the core of the optical fiber path havinga profile designed to allow useful light to propagate in a core modefrom the input end to the output end while coupling deleterious lightinto one or more counter-propagating cladding mode.
 18. The fiber lasercavity according to claim 17, wherein the refractive index modulationhas a period maximum at the input end and decreasing progressively fromthe input end to the output end.
 19. The fiber laser cavity according toclaim 18, wherein the deleterious light filter is positioned proximateone of the cavity mirrors, the input end facing towards a rest of thefiber laser cavity.
 20. The fiber laser cavity according to claim 17,wherein the refractive index modulation has a period varying along theFBG, the period being maximum at the input and output ends of the FBGand decreasing progressively towards an intermediate point thereof. 21.The fiber laser cavity according to claim 17, wherein the deleteriouslight comprises Raman scattered light.
 22. The fiber laser cavityaccording to claim 17, wherein the deleterious light comprises AmplifiedSpontaneous Emission.
 23. A fiber laser system, comprising: an opticalfiber path having a core and at least one cladding surrounding the core;a pair of cavity mirrors disposed in the optical fiber path and defininga fiber laser cavity therebetween; and a deleterious light filterdisposed along the optical fiber path, the deleterious light filtercomprising a Fiber Bragg grating (FBG) having an input end and an outputend, the FBG having a refractive index modulation in the core of theoptical fiber path having a profile designed to allow useful light topropagate from the input end to the output end while couplingdeleterious light into one or more counter-propagating cladding mode.24. The fiber laser system according to claim 23, wherein the refractiveindex modulation has a period maximum at the input end and decreasingprogressively from the input end to the output end.
 25. The fiber lasersystem according to claim 24, wherein the deleterious light filter isdisposed inside the fiber laser cavity.
 26. The fiber laser systemaccording to claim 25, wherein the deleterious light filter ispositioned proximate one of the cavity mirrors, the input end facingtowards a rest of the fiber laser cavity.
 27. The fiber laser systemaccording to claim 24, wherein the deleterious light filter is disposedoutside the fiber laser cavity.
 28. The fiber laser system according toclaim 27, wherein the deleterious light filter is positioned proximateone of the cavity mirrors, the input end facing towards the fiber lasercavity.
 29. The fiber laser system according to claim 23, wherein therefractive index modulation has a period varying along the FBG, theperiod being maximum at the input and output ends of the FBG anddecreasing progressively towards an intermediate point thereof.
 30. Thefiber laser system according to claim 29, wherein the deleterious lightfilter is disposed inside the fiber laser cavity.
 31. The fiber lasersystem according to claim 29, wherein the deleterious light filter isdisposed outside the fiber laser cavity.
 32. The fiber laser systemaccording to claim 23, wherein the deleterious light comprises Ramanscattered light.
 33. The fiber laser system according to claim 23,wherein the deleterious light comprises Amplified Spontaneous Emission.34. The fiber laser system according to claim 23, wherein the refractiveindex modulation of the FBG defines slanted grating fringes.
 35. Anoptical device supporting a light beam having a useful component and adeleterious component, the optical device comprising: an optical fiberpath having a core and at least one cladding, and configured to guidethe light beam in a core mode along a propagation direction; and a FiberBragg Grating (FBG) disposed along the optical fiber path and having aninput end and an output end with respect to the propagation direction,the FBG including a refractive index modulation in the core of theoptical fiber path configured to allow the useful component of the lightbeam through the FBG and to couple the deleterious component of thelight beam into one or more counter propagating cladding mode of theoptical fiber path.
 36. The optical device according to claim 35,wherein the refractive index modulation of the FBG has a chirped periodlarger at the input end than at the output end.
 37. The optical deviceaccording to claim 35, wherein the refractive index modulation has aperiod varying along the FBG, the period being maximum at the input andoutput ends of the FBG and decreasing progressively towards anintermediate point thereof.
 38. The optical device according to claim35, wherein the refractive index modulation of the FBG defines slantedgrating fringes having a tilt angle.
 39. The optical device according toclaim 38, wherein the tilt angle of the grating fringes is between about1.5 and 15 degrees.
 40. The optical device according to claim 38,wherein the tilt angle of the grating fringes is between about 2.6 and5.2 degrees.
 41. The optical device according to claim 35, wherein therefractive index modulation of the FBG defines grating fringes coveringa portion only of the core.
 42. The optical device according to claim35, wherein the grating fringes are normal to a longitudinal axis ofsaid core.
 43. The optical device according to claim 35, wherein thecore of the optical fiber path of the FBG is multimode.
 44. The opticaldevice according to claim 35, wherein the period of the refractive indexmodulation the FBG is linearly chirped.
 45. The optical device accordingto claim 35, wherein the period of the refractive index modulation ofthe FBG is nonlinearly chirped.
 46. The optical device according toclaim 35, wherein the refractive index modulation of the FBG has anapodized amplitude.
 47. The optical device according to claim 35,wherein the deleterious component is one of Amplified SpontaneousEmission or Raman scattering.
 48. The optical device according to claim35, further comprising an active tuning mechanism coupled to the FBG.49. The optical device according to claim 35, further comprising a lasercavity.
 50. The optical device according to claim 49, wherein the FBG ispositioned inside of the laser cavity.
 51. The optical device accordingto claim 49, wherein the FBG is positioned outside of the laser cavity.52. The optical device according to claim 35, further comprising a lightstripper configured to evacuate the deleterious component from the oneor more counter propagating cladding mode.